Recovered orphaned WIP before switching to a new task

Scaffolding only — FastAPI app, SQLAlchemy models, Alembic migrations,
and docker-compose from the original task description are still not
implemented. Committing what exists so it isn't lost; task ec0074121ca3
reset to pending to finish the rest.

backend/app/models/user.py

@@ -1,5 +1,7 @@
 from sqlalchemy import Column, Integer, String, Boolean, DateTime
+from sqlalchemy.orm import relationship
 from sqlalchemy.sql import func
+
 from app.database import Base
 
 
@@ -13,3 +15,7 @@ class User(Base):
     is_active = Column(Boolean, default=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+    sessions = relationship("Session", back_populates="user", cascade="all, delete-orphan")
+    worlds = relationship("World", back_populates="owner")
+    assets = relationship("Asset", back_populates="owner")